The Age of the Earth

THE close of one century, the dawn of another, may naturally suggest some brief retrospective glance over the path along which our science has advanced, and some general survey of its present position; but other connection with geology the beginnings and endings of centuries have none. The great periods of movement have hitherto begun, as it were, in the early twilight hours, long before the dawn. Thus the first step forward, since which there has been no retreat, was taken by Steno in the year 1669; more than a century elapsed before James Hutton (1785) gave fresh energy and better direction to the faltering steps of the young science; while it was less than a century later (1863) when Lord Kelvin brought to its aid the powers of the higher mathematics and instructed it in the teachings of modern physics. From Steno onward the spirit of geology was catastrophic; from Hutton onward it grew increasingly uniformitarian; from the time of Darwin and Kelvin it has become evolutional. The ambiguity of the word “uniformitarian “has led to a good deal of fruitless logomachy, against which it may be as well at once to guard by indicating the sense in which it is used here. In one way we are all uniformi- tarians, i. e., we accept the doctrine of the uniform action of natural causes,” but, as applied to geology, uniformity means more than this. Defined in the briefest fashion it is the geology of Lyell. Hutton had given us a “Theory of the Earth,” in its main outlines still faithful and true; and this Lyell spent his life in illustrating and advocating; but as so commonly happens, the zeal of the disciple outran the wisdom of the master, and mere opinions were insisted on as necessary dogma. What did it matter if Hutton as a result of his inquiries into terrestrial history had declared that he found no vestige of a beginning, no prospect of an end It would have been marvelous if he had! Consider that when Hutton's “Theory” was published, William Smith's famous discovery had not been made, and that nothing was then known of the orderly succession of forms of life, which it is one of the triumphs of geology to have revealed; consider, too, the existing state of physics at the time, and that the modern theories of energy had still to be formulated; consider, also, that spectroscopy had not yet lent its aid to astronomy and the consequent ignorance of the nature of nebulae; and then, if you will, cast a stone at Hutton. With Lyell, however, the,case was different. In pressing his uniformitarian creed upon geology, he omitted to take into account the great advances made by its sister sciences, although he had knowledge of them, and thus sinned against the light. In the last edition of the famous “Principles” we read: “It is a favorite dogma of some physicists that not only the earth, but the sun itself, is continually losing a portion of its heat, and that as there is no known source by which it can be restored, we can foresee the time when all life will cease to exist on this planet, and on the other hand we can look back to a period when the heat was so intense as to be incompatible with the existence of any organic beings such as are known to us in the living or fossil world.
Opening address by the President of the Section of Geology, British Association.
A geologist in search of some renovating power by which the amount of heat may be made to continue unimpaired for millions of years, past and future, in the solid parts of the earth... has been compared by an eminent physicist to one who dreams he can discover a source of perpetual motion and invent a clock with a self-winding apparatus. But why should we despair of detecting proofs of such regenerating and self-sustaining power in the works of a Divine Ar- tilicer Here we catch the true spirit of uniformity; it admittedly regards the universe as a self-winding clock, and barely conceals a conviction that the clock was warranted to keep true Greenwich time. The law of the dissipation of energy is not a dogma, but a doctrine drawn from observation, while the uniformity of Lyell is in no sense an induction; it is a dogma in the narrowest sense of the word, unproved, incapable of proof; hence perhaps its power upon the human mind; hence, also, the transitoriness of that power. Again, it is only by restricting its inquiries to the stratified rocks of our planet that the dogma of uniformity can be maintained with any pretense of argument. Directly we begin to search the heavens, the possibility, nay even the likelihood, of the nebular origin of our system, with all that it involves, is borne in upon us. Lyell therefore consistently refused to extend his gaze beyond the rocks beneath his feet, and was thus led to do a serious injury to our science. He severed it from cosmogony, for which he entertained and expressed the most profound contempt, and from the mutilation thus inflicted geology is only at length making a slow and painful recovery. Why do I dwell on these facts To depreciate Lyell By no means. No one is more conscious than I of the noble service which Lyell rendered to our cause; his reputation is of too robust a kind to suffer from my unskillful handling, and the fame of his solid contributions to science will endure long after these controversies are forgotten. The echoes of the combat are already dying away, and uni- formitarians, in the sense already defined, are now no more. Indeed, were I to attempt to exhibit any distinguished living geologist as a still surviving supporter of the narrow Lyellian creed, he would probably feel, if such a one there be, that I was unfairly singling him out for unmerited obloquy.
Our science has become evolutional, and in the transformation has grown more comprehensive; her petty parochial days are done, she is drawing her provinces closer around her, and is fusing them together into a united and single commonwealth–the science of the earth.
Not merely the earths crust, but the whole of earth- knowledge is the subject of our research. To know all that can be known about our planet, this, and nothing less than this, is its aim and scope. From the morphological side geology inquires, not only into the existing form and structure of tne earth, but also into the series of successive morphological states through which it has passed in a long and changeful development. Our science inquires also into the distribution of the earth in time and space; on the physiological side it studies the movements and activities of our planet; and not content with all this, it extends its researches into aetiology and endeavors to arrive at a science of causation. In these pursuits, geology calls all the other sciences to her aid. In our commonwealth there are no outlanders: if ati eminent physicist enter our territory, we do not begin at once to prepare for war, because the very fact of his undertaking a geological inquiry of itself confers upon him all the duties and privileges of citizenship. A physicist studying geology is by definition a geologist. Our only regret is, not that physicists occasionally invade our borders, but that they do not visit us oftener and make closer acquaintance with us.
EARLY HISTORY OP THE EARTH–FIRST CRITICAL PERIOD.
If I am bold enough to assert that cosmogony is no longer alien to geology, I may proceed further, and taking advantage of my temerity pass on to speak of things once not permitted to us. I propose, therefore, to offer some short account of the early stages in the history of the earth. Into its nebular origin we need not inquire–that is a subject for astronomers. We are content to accept the infant earth from their hands as a molten globe ready made, its birth from a gaseous nebula duly certified. If we ask, as a matter of curiosity, what was the origin of the nebula, I fear even astronomers cannot tell us. There is an hypothesis which refers it to the clashing of meteorites, but in the form in which this is usually presented it does not help us much. Such meteorites as have been observed to penetrate our atmosphere and to fall on to the surface of the earth prove on examination to have had an eventful history of their own, of which not the least important chapter was a passage through a molten state; they would thus appear to be the products rather than the progenitors of a nebula.
We commence our history, then, with a rapidly rotating molten planet, not impossibly already solidified about the center and surrounded by an atmosphere of great depth, the larger part of which was contributed by the water of our present oceans, then existiug in a state of gas. This atmosphere, which exerted a pressure of something like 5000 pounds to the square inch, must have played a very important part in the evolution of our planet. The molten exterior absorbed it to an extent which depended on the pressure, and which may some day be learnt from experiment. Under the influence of the rapid rotation of the earth, the atmosphere would be much deeper in equatorial than polar regions, so that in the latter the loss of heat by radiation would be in excess. This might of itself lead to convectional currents in the molten ocean. The effect on the atmosphere is very difficult to trace, but it is obvious that if a high-pressure area originated over some cooler region of the ocean, the winds blowing out of it would drive before them the cooler superficial layers of molten material; and as these were replaced by hotter lava streaming from below, the tendency would be to convert the high into a low-pressure area, and to reverse the direction of the winds. Conversely under a low-pressure area the in-blowing winds would drive in the cooler superficial layers of molten matter that had been swept away from the anticyclones. If the difference in pressure under the cyclonic and anti- cyclonic areas were considerable, some of the gas absorbed under the anticyclones might escape beneath the cyclones, and in a later stage of cooling might give rise to vast floating islands of scoria. Such islands might be the first foreshadowings of the future continents. Whatever the ultimate effect of the reaction of the winds on the currents of the molten ocean, it is probable that some kind of circulation was set up in the latter. The universal molten ocean was by no means homogeneous; it was constantly undergoing changes in composition as it reacted chemically with the internal metallic nucleus; its currents would streak the different portions out in directions which in the northern hemisphere would run from northeast to southwest, and thus the differences which distinguish particular penological regions of our planet may have commenced their existence at a very early stage. It is possible that as our knowledge extends we shall be able by a study of the distribution of igneous rocks and minerals to draw some conclusions as to the direction of these hypothetical lava currents Our planet was profoundly disturbed by tides, produced by the sun; for as yet there was no moon; and it has been suggested that one of its tidal waves rose to a height so great as to sever its connection with the earth and to fly off as the infant moon. This event may be regarded as marking the first critical, period, or catastrophe if we please, in the history of our planet. The career of our satellite, after its escape from the earth, is not known till it attained a distance of nine terrestrial radii; after this its progress can be clearly followed. At the eventful time of parturition the earth was rotating, with a period of from two to four hours, about an axis inclined at some 11° or 12° to the ecliptic. The time which has elapsed since the moon occupied a position nine terrestrial radii distant from the earth is at least fifty-six to fifty-seven millions of years, but may have been much more. Prof. Darwin's story of the moon is certainly one of the most beautiful contributions ever made by astronomy to geology, and we shall all concur with him when he says,A theory reposing on verse causae, which brings into quantitative correlation the length of the present day and month, the obliquity of the ecliptic, and the inclination and eccentricity of the lunar orbit, must, I think, have strong claims to acceptance.”
The majority of geologists have long hankered after a metallic nucleus for the earth, composed chiefly, by analogy with meteorites, of iron. Lord Kelvin has admitted the probable existence of some such nucleus, and lately Prof. Wiechert has furnished us with arguments–”powerful” arguments Prof. Darwin terms them–in support of its existence. The interior of the earth for four-fifths of the radius is composed, according to Prof. Wiechert, chiefly of metallic iron, with a density of 8 2; the outer envelope, one-fifth of the radius, or about 400 miles in thickness, consists of silicates, such as we are familiar with in igneous rocks and meteorites, and possesses a density of 3.2. It was from this outer envelope when molten that the moon was trundled off, twenty-seven miles in depth going to its formation. The density of this material, as we have just seen, is supposed to be 3.2; the dens t.v of the moon is 3 39, a close approximation, such difference as exists being completely explicable by the comparatively low temperature of the moon.
The outer envelope of the earth which was drawn off to form the moon was, as we have seen, charged with steam and other gases under a pressure of 5,000 pounds to the square inch; but as the satellite wandered away from the parent planet, this pressure continuously diminished. Under these circumstances the moon would become as explosive as a charged bomb, steam would burst forth from numberless volcanoes, and while the face of the moon might thus have acquired its existing features, the ejected material might possibly have been shot so far away from its origin as to have acquired an independent orbit. If so, we may ask whether it may not be possible that the meteorites, which sometimes descend upon our planet, are but portions of its own envelope returning to it. The facts that the average specific gravity of those meteorites which have been seen to Fall is not much above 3.2, and that they have passed through a stage of fusion, are consistent with this suggestion.
SECOND CRITICAL PERIOD–” CONSISTENTIOR STATUS.”
The solidification of the earth probably became completed soon after the birth of the moon. The temperature of its surface at the time of consolidation was about 1,170° C., and it was therefore still surrounded by its primitive deep atmosphere of steam and other gases. This was the second critical period in the history of the earth, the stage of the “consistentior status,” the date of which Lord Kelvin would rather know than that of the Norman Conquest, though he thinks it lies between twenty and forty millions of years ago, probably nearer twenty than forty.
Now that the crust was solid there was less reason why movements of the atmosphere should be unsteady, and definite regions of high and low pressure might have been established. Under the high-pressure areas the surface of the crust would be depressed; correspondingly under the low-pressure areas it would be raised; and thus from the first the surface of the solid earth might be dimpled and embossed.
It would be difficult to rtiscnps with sufficient brevity the probable distribution of these inequalities, hut it may be pointed out that the moon is possibly responsible, anrt that in more ways than one, for much of the exicsting geographical asymmetry.
THIRD CRITICAL PERIOD–ORIGIN OF THE OCEANS.
The cooling of the earth would continuously progress, till the temperature of the surface fell to 370° C., when that part of the atmosphere which consisted of steam would begin to liquefy; then the dimples on the surface would soon become filled with superheated water, and the pools so formed would expand and deepen, till they formed the oceans. This is the third critical stage in the history of the earth, dating, according to Prof. Joly, from between eighty and ninety millions of years ago. With the growth of the oceans the distinction between land ana sea arose–in what precise manner we may proceed to inquire. If we revert to the period of the consistentior status,” when the earth had just solidified, we shall find, according to Lord Kelvin, that the temperature continuously increased from the surface, where it was 1,170° C., down to a depth of twenty-five miles, where it was about 1,430° C., or 260&ast; C. above the fusion point of the matter forming the crust. That the crust at this depth was not molten but solid is to be explained by the very great pressure to which it was subjected–just so much pressure, indeed, as was required to counteract the influence of the additional 260° C. Thus if we could have reduced the pressure on the crust we should have caused it to liquefy; by restoring the pressure it would resolidify. By the time the earths surface had cooled down to 370° C. the depth beneath the surface at which the pressure just kept the crust solid would have sunk some slight distance inward, but not sufficiently to affect our argument.
The average pressure of the primitive atmosphere upon the crust can readily be calculated by supposing the water of the existing oceans to be uniformly distributed over the earth's surface, and then by a simple piece of arithmetic determining its depth; this is found to be 1.718 miles, the average depth of the oceans being taken at 2'393 miles. Thus the average pressure over the earths surface, immediately before the formation of the oceans, was equivalent to that of a column of water 1.718 miles high on each square inch. Supposing that at its origin the ocean were all gathered together into one place,” and “the dry land appeared,” then the pressure over the ocean floor would be increased from 1.718 miles to 2.393 miles, while that over those portions of the crust that now formed the land would be diminished by 1.718 miles. This difference in pressure would tend to exaggerate those faint depressions which had arisen under the primitive anti-cyclonic areas, and if the just soldified material of the earth's cruat were set into a state of flow, it might move from under the ocean into the buildings which were rising to form the land, until static equilibrium were established. Under these circumstances the pressure of the ocean would be just able to maintain a column of rock 0886 mile in height, or ten twenty-sevenths of its own depth. It could do no more; but in order that the dry land may appear, some cause must be found competent either to lower the ocean bed the remaining seventeen twenty-sevenths of its full depth, or to raise the continental buildings to the same extent. Such a cause may, I think, be discovered in a further effect of the reduction in pressure over the continental areas. Previous to the condensation of the ocean, these, as we have seen, were subjected to an atmospheric pressure equal to that of a column of water 1/718 miles in height. This pressure was contributory to that which caused the outer twenty-five miles of the earth's crust to become solid; it furnished, indeed, just about one- fortieth of that pressure, or enough to raise the fusion point 6° C. What, then, might be expected to happen when the continental area was relieved of this load Plainly a liquefaction and corresponding expansion of the underlying rock
But we will not go so far as to assert that actual liquefaction would result; all we require for our explanation is a great expansion; and this would probably follow, whether the crust were liquefied or not. For there is good reason to suppose that when matter at a temperature above its ordinary fusion point is compelled into the solid state by pressure, its volume
is very responsive to changes either of pressure or temperature. The remarkable expansion of liquid carbon dioxide is a case in point: 120 volumes of this fluid at –20° C. become 150 volumes at 33° G.; a temperature just below the critical point. A great change of volume also occurs when the material of igneous rocks passes from the crystalline state to that of glass; in the case of diabase the difference in volume of the rock in the two states at ordinary temperatures is 13 per cent. If the relief of pressure over the site of continents were accompanied by volume changes at all approaching this, the additional elevation of seventeen twenty- sevenths required to raise the land to the sea level would be accounted for.f How far down beneath the surface the unloading of the continents would be felt it is difficult to say, though the problem is probably not beyond the reach of mathematical analysis; if it affected an outer envelope twenty-five miles in thickness, a linear expansion of 4 per cent, would suffice to explain the origin of ocean basins. If now we refer to the dilatation determined by Carl Barus for rise in temperature in the case of diabase, we find that between 1,093° and 1,112° C. the increase in volume is 3.3 per cent. As a further factor in deepening the ocean asins may be included the compressive effect of the increase in load over the ocean floor; this increase is equal to the pressure of a column of water 0'675 mile in height, and its effect in raising the fusion point would be 2° C., from which we may gain some kind of idea of the amount of compression it might produce on the yielding interior of the crust. To admit that these views are speculative will be to confess nothing; but they certainly account for a good deal. They not only give us ocean basins, but basins of the kind we want, that is, to use a crude comparison once made by the late Dr. Carpenter, basins of a tea- tray form, having a somewhat flat floor and steeply sloping sides; they also help to explain how it is that the value of gravity is greater over the ocean than over the land.
C. Harus.o names the material on which he experimented; apparently the rock is a french dolerite without olivine..
Prof. Fitzgerald has been kind enougb to express part of the preceding exphuultion in a more precise manner for me. He writes: H It would reire very uice adjustment of tempenltures and pressure. to work out. in t Ie simple way you state it; but what is really involved is that in a certain tate dIabase (and everything that chance state with a considerable chae of volume) hus an enorlllous isothermal compressibility. Although this is very in the case of botlics which melt suddenly, like ice, it would also involve very great compreAsibilities in the case of bodies even which melted gradually. if they did so at all quickly. i. e, within a small range of temperature. What you postulate, tben, is tbat at a certam depth diabase is soft enough to be squcezerl from under the oceans, anlt that, being near its melting point, tbe small relief of pressure i. accompanied by all enormous Increase i ll volnme which helped to raise tbe continente. Now that I have written the thing out In my own way, it seems very likely. It is, anyway, a suggestion quite worthy of.erious consideration. and a process that in some places,nust almost certainly have been in operation, and maybe is still operative. Looking at It agaill, I hardly think it is quite likely thllt there is or conld be much squeezin2 sideway. of liquid or other. vicous material from under one place to another, ht,cullSe the e1c yielding of the inside of the earth would be much quicker tban any dow of this kmd. This wouhl ollly modif; yonr theory, becayse the diabase that expands so much on the relief 0 pres.ure might be that already under the land, and,i.ing np this latter. partly by being pushed up itself by the cla.tic rel ief 01 the inside of the earth and partly by its own enormous expnnsihil ity near itg melting point. The actIon would be qui slow, because It wouhl cool itse l f so much by its expansion that it would have to be warmcd up from below, or b.lulal enrthaqneezinor by chemical action, before it could explain normally
The ocean when first formed would consist of highly heated water, and this, as is well known, is an energetic chemical reagent when brought into contact with silicates like those which formed the primitive crust. As a result of its action, saline solutions and chemical deposits would be formed; the latter, however, would probably be of no great thickness, for the time occupied by the ocean in cooling to a temperature not far removed from the present would probably be included within a few hundreds of years.

This article was originally published with the title "The Age of the Earth"

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